The present invention relates to a multicellular model comprising keratinocytes, melanocytes and nerve cells, and also to the use of this model for the purposes of screening for active agents, in particular with regard to melanogenesis.
Numerous cell models aimed at simulating the various properties and characteristics of the skin had been developed in order to make it possible to carry out studies necessary for a better understanding of the role of the various elements constituting the skin, both in mechanical terms and in physiological terms.
In general, these models comprise human keratinocytes deposited onto a support, for example a dermis equivalent, and cultured under conditions suitable for the formation of an epidermal equivalent.
However, the skin is a complex structure comprising various cell types, where appropriate, organized in tissue structures, within which and between which cellular communications that regulate skin homeostasis can be established.
By way of example, skin comprises, in particular, keratinocytes, melanocytes and nerve cells, but also fibroblasts, Langerhans cells, endothelial cells constituting the blood vessels that irrigate the dermis and the hypodermis, smooth muscle cells, etc.
The keratinocytes are mainly responsible for the production of keratin, one of the essential constituents of the horny layer. They are continually undergoing a morphological evolution that is a reflection of their keratinization underlying the protective (mechanical, chemical) barrier role of the epidermis.
The melanocytes are located in the basal layer of the epidermis, they are the site of melanogenesis and, due to their close contact with the keratinocytes, they transfer to the latter the newly synthesized melanin in the form of melanosomes, thus giving the skin its coloration.
The type and the amount of melanin contained in the melanosomes determine the coloration of the skin. Melanin constitutes, in particular, an effective protective screen against solar radiation, in particular ultraviolet radiation.
Melanogenesis is a complex biological phenomenon initiated by hydroxylation of the L-tyrosine amino acid resulting from the formation of L-dihydroxyphenylalanine (L-DOPA), which is in turn converted to DOPA-chrome by the action of a specific melanocyte-associated enzyme, tyrosinase. Consecutive reduction and oxidation reactions result in the conversion of the DOPA-chrome to melanin. The production of tyrosinase and its activity determine in part the amount of melanin produced. The amount and the type of melanin transferred to the keratinocytes determine, for their part, the degree of visual pigmentation of human skin.
Now, melanin can be synthesized excessively, or even anarchically, in response to an exogenous stress such as pollution and UV rays, and/or an endogenous stress, for example due to ageing of the keratinocytes, the endothelial cells, the fibroblasts and the Langerhans cells.
Thus, many skin disorders can result from a disruption of melanogenesis and result, for example, in an overload of melanin or in an abnormal distribution of melanin in the skin, called hypermelanosis. In hypermelanoses, melanoderma, an anomaly associated with the epidermis, and ceruloderma, a dermal anomaly, can be distinguished.
Recently, it has been noted that emotional stress, for example of neurogenic origin or involving nerve cells, can induce a release of hormones and of neurohormones capable of affecting the homeostasis of melanogenesis.
Now, to date, none of the available cell models makes it possible to reproduce and study the impact of nerve activity, satisfactorily, on the physiological functions of the skin, in particular on melanogenesis.
The present invention is aimed at precisely satisfying this need.